Spin-Torque-Triggered Magnetization Reversal in Magnetic Tunnel Junctions with Perpendicular Shape Anisotropy

Abstract

Recently, the concept of perpendicular shape anisotropy spin-transfer-torque magnetic random-access memory (PSA STT MRAM) was proposed to enhance the thermal stability factor, $\mathrm{\ensuremath{\Delta}}$, of the storage layer through its shape anisotropy, thus enabling the downsizing scalability of conventional STT MRAM beyond sub-20-nm technological nodes. However, this is expected to negatively impact the writing current and switching time, calling for the search for the best compromise. Here, we report a micromagnetic study of the STT-driven magnetization reversal of sub-20-nm PSA STT MRAM cells, with a particular emphasis on the technologically relevant case of $\mathrm{\ensuremath{\Delta}}\ensuremath{\simeq}80$, thus providing guidelines for their practical design. For an aspect ratio ($\ensuremath{\eta}$) of the storage layer of $\ensuremath{\eta}\ensuremath{\le}1$, magnetization reversal obeys a macrospinlike mechanism, while for $\ensuremath{\eta}>1$ a noncoherent reversal is observed, which is characterized by buckling or nucleation of a transverse domain wall at the ferromagnet-insulator interface, which then propagates along the vertical axis. Decreasing the lateral size while maintaining a constant value of $\mathrm{\ensuremath{\Delta}}$ implies a larger height, which is found to lead to an increase of the switching voltage. In all cases, the inverse of the switching time scales linearly with the applied voltage.